Multimode waveguide bends with features to reduce bending loss

ABSTRACT

Structures for a waveguide bend and methods of fabricating a structure for a waveguide bend. A waveguide core has a first section, a second section, and a waveguide bend connecting the first section with the second section. The waveguide core includes a first side surface and a second side surface, the first side surface extends about an inner radius of the waveguide bend, and the second side surface extends about an outer radius of the waveguide bend. The waveguide bend includes a central region and a side region that is arranged adjacent to the central region at the first side surface or the second side surface. The central region has a first thickness, and the side region has a second thickness that is less than the first thickness.

BACKGROUND

The present invention relates to photonics chips and, more specifically,to structures for a waveguide bend and methods of fabricating astructure for a waveguide bend.

Photonics chips may be used in many applications and systems including,but not limited to, data communication systems and data computationsystems. A photonics chip integrates optical components, such aswaveguides, optical switches, resonators, directional couplers, andwaveguide bends, and electronic components, such as field-effecttransistors, into a monolithic platform. Among other factors, layoutarea, cost, and operational overhead may be reduced by the integrationof both types of components on the same monolithic platform.

Waveguide bends may be employed on a photonics chip to change thepropagation direction of optical signals propagating in a waveguidecore. Bending losses from inter-modal mixing and interference are anubiquitous problem in multimode waveguide bends and may perturb thefundamental mode of the optical signal. Bending losses may be minimizedby fabricating a waveguide bend with a large bending radius ranging fromhundreds of microns to a millimeter or larger. However, the large sizeof such waveguide bends reduces the available space on the photonicschip for other optical and electronic components.

Improved structures for a waveguide bend and methods of fabricating astructure for a waveguide bend are needed.

SUMMARY

In an embodiment of the invention, a structure includes a waveguide corehaving a first section, a second section, and a waveguide bendconnecting the first section with the second section. The waveguide coreincludes a first side surface and a second side surface, the first sidesurface extends about an inner radius of the waveguide bend, and thesecond side surface extends about an outer radius of the waveguide bend.The waveguide bend includes a central region and a side region that isarranged adjacent to the central region at the first side surface or thesecond side surface. The central region has a first thickness, and theside region has a second thickness that is less than the firstthickness.

In an embodiment of the invention, a structure includes a waveguide coreincluding a first section, a second section, and a waveguide bendconnecting the first section with the second section. The waveguide coreincludes a first side surface and a second side surface, the first sidesurface extends about an inner radius of the waveguide bend, and thesecond side surface extends about an outer radius of the waveguide bend.The structure further includes a curved strip arranged over thewaveguide bend adjacent to the first side surface or the second sidesurface.

In an embodiment of the invention, a method includes patterning a firstlayer to form a waveguide core having a first section, a second section,and a waveguide bend that connects the first section with the secondsection. The method further includes forming an etch mask over the firstsection of the waveguide core, the second section of the waveguide core,and a central region of the waveguide bend that exposes a side region ofthe waveguide bend adjacent to the central region of the waveguide bendat a side surface of the waveguide core. The method further includesetching partially through the side region of the waveguide bend with theetch mask over the first section of the waveguide core, the secondsection of the waveguide core, and the central region of the waveguidebend. The central region of the waveguide bend has a first thickness,and the side region of the waveguide bend has a second thickness that isless than the first thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate various embodiments of theinvention and, together with a general description of the inventiongiven above and the detailed description of the embodiments given below,serve to explain the embodiments of the invention. In the drawings, likereference numerals refer to like features in the various views.

FIG. 1 is a diagrammatic top view of a structure at an initialfabrication stage of a processing method in accordance with embodimentsof the invention.

FIG. 2 is a cross-sectional view of the structure taken generally alongline 2-2 in FIG. 1.

FIG. 2A is a cross-sectional view of the structure taken generally alongline 2A-2A in FIG. 1.

FIG. 2B is a cross-sectional view of the structure taken generally alongline 2B-2B in FIG. 1.

FIG. 2C is a cross-sectional view of the structure taken generally alongline 2C-2C in FIG. 1.

FIGS. 3, 3A, 3B, 3C are cross-sectional views of the structure of FIGS.2, 2A, 2B, 2C at a subsequent fabrication stage of the processingmethod.

FIGS. 4 and 5 are cross-sectionals view of structures in accordance withalternative embodiments of the invention.

FIG. 6 is a top view of a structure at an initial fabrication stage of aprocessing method in accordance with embodiments of the invention.

FIG. 7 is a cross-sectional view of the structure taken generally alongline 7-7 in FIG. 6.

FIG. 7A is a cross-sectional view of the structure taken generally alongline 7A-7A in FIG. 6.

FIG. 7B is a cross-sectional view of the structure taken generally alongline 7B-7B in FIG. 6.

FIG. 7C is a cross-sectional view of the structure taken generally alongline 7B-7B in FIG. 6.

FIGS. 8, 8A, and 8B are cross-sectional views of structures inaccordance with alternative embodiments of the invention.

FIGS. 9, 9A, and 9B are cross-sectional views of structures inaccordance with alternative embodiments of the invention.

FIGS. 10, 10A, and 10B are cross-sectional views of structures inaccordance with alternative embodiments of the invention.

DETAILED DESCRIPTION

With reference to FIGS. 1, 2, 2A, 2B, 2C and in accordance withembodiments of the invention, a structure 10 includes a waveguide core12 having a section 14 that extends symmetrically along a longitudinalaxis 13, section 16 that extends symmetrically along a longitudinal axis15, and a waveguide bend 18 that are arranged over a dielectric layer20. The longitudinal axis 13 of the section 14 of the waveguide core 12is angled or inclined at an angle relative to the longitudinal axis 15of the section 16 of the waveguide core 12 due to a change in directionprovided by the waveguide bend 18. In an embodiment, the angle may be aright angle (i.e., 90°). The waveguide core 12 includes sidewalls in theform of side surfaces 22, 24 that project in a vertical direction fromthe dielectric layer 20. The side surfaces 22, 24 may be substantiallylinear or straight over the sections 14, 16 of the waveguide core 12,and the side surfaces 22, 24 may be non-linear or curved over thewaveguide bend 18 of the waveguide core 12. In particular, the sidesurface 22 of the waveguide core 12 extends about the inner edge at aninner radius of the waveguide bend 18, and the side surface 24 of thewaveguide core 12 extends about the outer edge at the outer radius ofthe waveguide bend 18.

The waveguide core 12 may be composed of a single-crystal semiconductormaterial, such as single-crystal silicon and, in particular,single-crystal silicon originating from a device layer of asilicon-on-insulator (SOI) wafer. The silicon-on-insulator wafer furtherincludes a buried insulator layer composed of a dielectric material,such as silicon dioxide, that provides the dielectric layer 20 and asubstrate 26 composed of a single-crystal semiconductor material, suchas single-crystal silicon, under the buried insulator layer. Thewaveguide core 12 may be patterned from a layer of single-crystalsemiconductor material (e.g., the device layer of the SOI wafer) bylithography and etching processes during front-end-of-line processing.

To that end, initial shapes for the waveguide bend 18 and sections 14,16 of the waveguide core 12 are patterned with a lithography and etchingprocess from the layer of single-crystal semiconductor material. Thecurved initial shape of the waveguide bend 18 may be the same width andhave the same thickness as the initial shapes of the sections 14, 16. Anetch mask is applied that fully masks the initial shapes of the sections14, 16 and that only partially masks the initial shape of the waveguidebend 18. In the latter regard, the etch mask over the initial shape ofthe waveguide bend 18 covers the central portion 32 of the waveguidebend 18 from end 19 to end 21 and exposes the side regions 28, 30 of thewaveguide bend 18 from end 19 to end 21. An etching process is thenused, with the etch mask present, to reduce the thickness of the exposedside regions 28, 30 of the waveguide bend 18. In the representativeembodiment, both exposed side regions 28, 30 of the waveguide bend 18are thinned. The etch mask is stripped after thinning the side regions28, 30 of the waveguide bend 18.

Due to the partial-thickness etching, the waveguide bend has the form ofa ridge waveguide that includes peripheral or side regions 28, 30 ofreduced thickness extending outwardly from the base of a central region32. The central region 32 of the waveguide bend 18 may have a narrowerwidth than either of the sections 14, 16 of the waveguide core 12. Thetotal width of the regions 28, 30, 32 of the waveguide bend 18 may besubstantially equal to the width of the section 14 of the waveguide core12 and/or the section 16 of the waveguide core 12. The sections 14, 16of the waveguide core 12 have the form of rib waveguides, as shown, forwhich the layer of single-crystal semiconductor material is fully etchedto expose the dielectric layer 20 about the sections 14, 16 of thewaveguide core 12. The sections 14, 16 of the waveguide core 12 and thecentral region 32 of the waveguide bend 18 may have substantially equalthicknesses, which may be approximately equal to the thickness of thepatterned layer of single-crystal semiconductor material. The sideregions 28, 30 have a thickness, t1, that is less than the thickness,t2, of the central region 32 of the waveguide bend 18 and the sections14, 16 of the waveguide core 12. In an embodiment, the thickness of theside regions 28, 30 may range from about 10% of the thickness of thecentral region 32 to about 60% of the thickness of the central region32.

The waveguide bend 18 has an end 19 that is directly connected andcontinuous with the section 14 of the waveguide core 12 to provide aseamless transition. The waveguide bend 18 has an opposite end 21 thatis directly connected and continuous with the section 16 of thewaveguide core 12 to provide a seamless transition. The end 19 of thewaveguide bend 18 may provide an input port coupled with the section 14,and the end 21 of the waveguide bend 18 may provide an output portcoupled with the section 16. The arc shapes of the side regions 28, 30and central region 32, which extend between the opposite ends 19, 21,may share the same central angle of curvature. The side regions 28, 30introduce respective indents in the curved portions of the side surfaces22, 24 associated with the waveguide bend 18 and may extend over thefull arc length of the waveguide bend 18 between the opposite ends 19,21 of the waveguide bend 18. The side regions 28, 30 may terminate atthe opposite ends 19, 21 of the waveguide bend 18. In the representativeembodiment, the side regions 28, 30 of the waveguide bend 18 aresymmetrically arranged relative to the central region 32 of thewaveguide bend 18.

The side regions 28, 30, which are arranged at respective sides of thecentral region 32 of the waveguide bend 18 in the representativeembodiment, provide features introducing additional perturbations thatmodify the properties of higher-order modes of an optical signalpropagating in the waveguide core 12 such that mode mixing with thefundamental mode of the propagating optical signal can be significantlyminimized. Specifically, the fundamental mode (e.g., TE0) guided in thewaveguide bend 18 may be preserved such that insertion loss is low,while other modes (e.g., TE1, TE2, etc.) guided in the waveguide bend 18may have altered profiles and altered effective indices that reducecrosstalk. This minimization in mode mixing may result in a reduction inthe bending loss exhibited by the waveguide bend 18.

With reference to FIGS. 3, 3A, 3B, 3C in which like reference numeralsrefer to like features in FIGS. 2, 2A, 2B, 2C and at a subsequentfabrication stage, dielectric layers 34, 36, 38, 40 composed ofrespective dielectric materials are sequentially formed in a layer stackover the waveguide core 12. In the layer stack, the dielectric layer 34is arranged over the dielectric layer 20, the dielectric layer 36 isarranged over the dielectric layer 34, the dielectric layer 38 isarranged over the dielectric layer 36, and the dielectric layer 40 isarranged over the dielectric layer 38. The waveguide core 12 is embeddedor buried in the dielectric material of the dielectric layer 34, whichacts as lateral cladding. The dielectric layer 34 may be composed of adielectric material, such as silicon dioxide, deposited by chemicalvapor deposition and planarized with, for example, chemical mechanicalpolishing to remove topography. The dielectric layer 36 may be composedof dielectric material, such as silicon dioxide, deposited by chemicalvapor deposition or atomic layer deposition over the dielectric layer34. The dielectric layer 38 may be composed of dielectric material, suchas silicon nitride, deposited by chemical vapor deposition or atomiclayer deposition over the dielectric layer 36. The dielectric layer 40may be composed of dielectric material, such as silicon dioxide,deposited by chemical vapor deposition or atomic layer deposition overthe dielectric layer 38. The dielectric layers 36, 38, 40 may be planarlayers arranged in the layer stack over the planarized top surface ofthe dielectric layer 34.

A dielectric layer 42 of a contact level is formed by middle-of-lineprocessing over the dielectric layer 40. The dielectric layer 42 may becomposed of dielectric material, such as silicon dioxide, deposited bychemical vapor deposition using ozone and tetraethylorthosilicate (TEOS)as reactants.

A back-end-of-line stack, generally indicated by reference numeral 44,is formed by back-end-of-line processing over the dielectric layer 42and the structure 10. The back-end-of-line stack 44 may include one ormore interlayer dielectric layers composed of one or more dielectricmaterials, such as a carbon-doped silicon oxide, and metallizationcomposed of, for example, copper, tungsten, and/or cobalt that isarranged in the one or more interlayer dielectric layers.

In alternative embodiments, the waveguide core 12 may be fabricatedusing a different material, such as polysilicon or silicon nitride. Inthis instance, the waveguide core 12 may be arranged over one or more ofthe dielectric layers 34, 36, 38, 40.

With reference to FIG. 4 and in accordance with alternative embodimentsof the invention, the side region 28 of the waveguide bend 18 may beomitted from the waveguide core 12 so as to provide an asymmetricalarrangement. As a result, the central region 32 of the waveguide bend 18may be enlarged in cross-sectional area, and the side region 30 of thewaveguide bend 18 may project radially outwardly from the base of thecentral region 32 of the waveguide bend 18 at the side surface 24 of thewaveguide core 12 extending about the outer radius of the waveguide bend18.

With reference to FIG. 5 and in accordance with alternative embodimentsof the invention, the side region 30 may be omitted from the structure10 so as to provide a different asymmetrical arrangement. As a result,the central region 32 of the waveguide bend 18 may be enlarged incross-sectional area, and the side region 28 of the waveguide bend 18may project radially outwardly from the base of the central region 32 ofthe waveguide bend 18 at the side surface 22 of the waveguide core 12extending about the inner radius of the waveguide bend 18.

With reference to FIGS. 6, 7, 7A, 7B, 7C and in accordance withalternative embodiments of the invention, a structure 54 may includecurved strips 46, 48 that are be arranged in a vertical direction overopposite sides of the waveguide bend 18. In the structure 54, thesections 14, 16 and the waveguide bend 18 of waveguide core 12 may havea uniform or constant width. The curved strips 46, 48 are contained in aplane that displaced vertically from a plane containing the waveguidebend 18. The curved strips 46, 48 may be composed of a differentmaterial than the waveguide core 12. In an embodiment, the curved strips46, 48 may be composed of silicon nitride that is deposited andpatterned with lithography and etching processes.

The curved strips 46, 48 may each be terminated, along the respectivecurve arcs, at an end 50 and at an end 52 that is opposite from the end50. The end 50 may be arranged proximate to, and over, the seamlesstransition in the waveguide core 12 from the section 14 to the waveguidebend 18. The end 52 may be arranged proximate to, and over, the seamlesstransition in the waveguide core 12 from the section 16 to the waveguidebend 18.

One of the curved strips 46 may be arranged adjacent to the side surface22 of the waveguide core 12 extending about the inner radius of thewaveguide bend 18, and the other of the curved strips 48 may be arrangedon the opposite side of the waveguide bend 18 adjacent to the sidesurface 24 of the waveguide core 12 extending about the outer radius ofthe waveguide bend 18. The curved strip 46 has an inner side surface 45defining an inner edge at an inner radius that is less than the innerradius of the waveguide bend 18 and an outer side surface 47 defining anouter edge at an outer radius that may be equal to the inner radius ofthe waveguide bend 18. The side surfaces 45, 47 are joined by the ends50, 52 of the curved strip 46. Similarly, the curved strip 48 has aninner side surface 49 with an inner radius that may be equal to theouter radius of the waveguide bend 18 and an outer side surface 51 at anouter radius that is greater than the outer radius of the waveguide bend18. The side surfaces 49, 51 are joined by the ends 50, 52 of the curvedstrip 48. In an embodiment, the curved strips 46, 48 may have the samecurvature as the respective side surfaces 22, 24 of the waveguide bend18, and may be symmetrically positioned relative to a curved centerlineof the waveguide bend 18.

Processing continues to form the dielectric layer 42, in which thecurved strips 46, 48 are embedded, and the back-end-of-line stack 44over the structure 54.

In an alternative embodiment, one or more additional curved strips maybe added to the curved strips 46, 48. For example, another curved stripmay be arranged interior of the inner radius of the curved strip 46,another curved strip may be arranged interior of the outer radius of thecurved strip 48, or both additional curved strips may be added incombination.

The curved strips 46, 48 provide features introducing additionalperturbations that modify the properties of higher-order modes of anoptical signal propagating in the waveguide core 12 such that modemixing with the fundamental mode of the propagating optical signalguided by the waveguide bend 18 can be significantly minimized.Specifically, the fundamental mode (e.g., TE0) guided by the waveguidebend 18 may be preserved such that insertion loss is low, while othermodes (e.g., TE1, TE2, etc.) guided by the waveguide bend 18 may havealtered profiles and altered effective indices that reduce crosstalk.This minimization in mode mixing may result is a reduction in thebending loss exhibited by the waveguide bend 18.

With reference to FIGS. 8, 8A, 8B in which like reference numerals referto like features in FIGS. 7, 7A, 7B and in accordance with alternativeembodiments of the invention, the curved strips 46, 48 may each havewidths that vary as a function of position along their curved lengths.The width, w1, of the curved strips 46, 48 adjacent to their oppositeends 50, 52 may be greater than the width, w2, of the curved strips 46,48 at locations between the opposite ends 50, 52. Processing continuesto form the dielectric layer 42, in which the curved strips 46, 48 areembedded, and the back-end-of-line stack 44 over the structure 54.

The width variation may be gradual and may progressively change by smallincrements, particularly near the opposite ends 50, 52 of the curvedstrips 46, 48. The width variation may be effective to reduce insertionloss.

With reference to FIGS. 9, 9A, 9B in which like reference numerals referto like features in FIGS. 2, 2A, 2B and in accordance with alternativeembodiments of the invention, the side regions 28, 30 of the waveguidebend 18 may be modified to have varying widths. The width, w3, of theside regions 28, 30 adjacent to the opposite ends of the waveguide bend18 may be greater than the width, w4, of the side regions 28, 30 48 atlocations between the opposite ends of the waveguide bend 18. Processingcontinues to form the dielectric layer 42 and the back-end-of-line stack44 over the structure 10.

The width variation may be gradual and may progressively change by smallincrements, particularly near the opposite terminating ends 19, 21 ofthe waveguide bend 18 at which the side regions 28, 30 terminate. Thewidth variation may be effective to reduce insertion loss.

With reference to FIGS. 10, 10A, 10B in which like reference numeralsrefer to like features in FIGS. 2, 2A, 2B and in accordance withalternative embodiments of the invention, the waveguide bend 18including the side regions 28, 30 may be used in combination with thecurved strips 46, 48 to provide a hybrid or composite structure 56. Thecurved strip 46 is displaced in a vertical direction (i.e., in they-direction in the x-z plane) from the side region 28, and the curvedstrip 48 is displaced in the vertical direction from the side region 30.The dielectric layers 36, 38, 40 are arranged in a vertical directionbetween the side regions 28, 30 and the curved strips 46, 48. The curvedstrip 46 may be arranged to overlap in a lateral direction with the sideregion 28 of the waveguide bend 18, and the curved strip 48 may bearranged to overlap in a lateral direction with the side region 30 ofthe waveguide bend 18. In an embodiment, the overlap may be completesuch that the side region 28 is fully arranged in a lateral direction(i.e., in the z-direction in the x-z plane) between the inner sidesurface 45 and the outer side surface 47 of the curved strip 46, and theside region 30 is fully arranged in the lateral direction between theinner side surface 49 and the outer side surface 51 of the curved strip48.

Processing continues to form the dielectric layer 42, in which thecurved strips 46, 48 are embedded, and the back-end-of-line stack 44over the structure 10.

In alternative embodiments of the composite structure 56, both of thecurved strips 46, 48 may be used in combination with the side region 28or with the side region 30, only the curved strip 46 may be used incombination with the side region 28 or with the side region 30, or onlythe curved strip 48 may be used in combination with the side region 28or with the side region 30. In addition, the widths of the curved strips46, 48 and/or the widths of the side regions 28, 30 may be varied in thecomposite structure 56 based on these combinations.

The structures 10, 54, 56, in any of the embodiments described herein,may be integrated into a photonics chip that includes electroniccomponents 60 (FIG. 1) and additional optical components 62 (FIG. 1)formed on the same chip to provide a monolithic platform. For example,the electronic components 60 may include field-effect transistors thatare fabricated by CMOS front-end-of-line (FEOL) processing, and theoptical components 62 may include resonators, optical switches, anddirectional couplers, as well as additional waveguides that may or maynot include bends.

The methods as described above are used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (e.g., as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. Thechip may be integrated with other chips, discrete circuit elements,and/or other signal processing devices as part of either an intermediateproduct or an end product. The end product can be any product thatincludes integrated circuit chips, such as computer products having acentral processor or smartphones.

References herein to terms modified by language of approximation, suchas “about”, “approximately”, and “substantially”, are not to be limitedto the precise value specified. The language of approximation maycorrespond to the precision of an instrument used to measure the valueand, unless otherwise dependent on the precision of the instrument, mayindicate +/−10% of the stated value(s).

References herein to terms such as “vertical”, “horizontal”, etc. aremade by way of example, and not by way of limitation, to establish aframe of reference. The term “horizontal” as used herein is defined as aplane parallel to a conventional plane of a semiconductor substrate,regardless of its actual three-dimensional spatial orientation. Theterms “vertical” and “normal” refer to a direction perpendicular to thehorizontal, as just defined. The term “lateral” refers to a directionwithin the horizontal plane.

A feature “connected” or “coupled” to or with another feature may bedirectly connected or coupled to or with the other feature or, instead,one or more intervening features may be present. A feature may be“directly connected” or “directly coupled” to or with another feature ifintervening features are absent. A feature may be “indirectly connected”or “indirectly coupled” to or with another feature if at least oneintervening feature is present. A feature “on” or “contacting” anotherfeature may be directly on or in direct contact with the other featureor, instead, one or more intervening features may be present. A featuremay be “directly on” or “in direct contact with” another feature ifintervening features are absent. A feature may be “indirectly on” or “inindirect contact with” another feature if at least one interveningfeature is present.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A structure comprising: a waveguide coreincluding a first section, a second section, and a waveguide bendconnecting the first section with the second section, the waveguide coreincluding a first side surface and a second side surface, the first sidesurface extending about an inner radius of the waveguide bend and thesecond side surface extending about an outer radius of the waveguidebend, the waveguide bend including a central region and a first sideregion that is arranged adjacent to the central region at the first sidesurface or the second side surface, the central region having a firstthickness, the first side region having a second thickness that is lessthan the first thickness, the first side region having a first endconnected with the first section and a second end connected with thesecond section, the first side region terminating at the first end, andthe first side region terminating at the second end; and a first curvedstrip arranged over the first side region of the waveguide bend, whereinthe first curved strip comprises silicon nitride, and the waveguide bendcomprises single-crystal silicon.
 2. The structure of claim 1 whereinthe first side region of the waveguide bend is arranged at the firstside surface, the waveguide bend includes a second side region arrangedat the second side surface, and the second side region of the waveguidebend has the second thickness.
 3. The structure of claim 2 wherein thefirst side region of the waveguide bend has a first width that varies asa function of position along the first side surface, and the second sideregion of the waveguide bend has a second width that varies as afunction of position along the second side surface.
 4. The structure ofclaim 2 further comprising: a second curved strip arranged over thesecond side region of the waveguide bend, wherein the second curvedstrip comprises silicon nitride.
 5. The structure of claim 1 furthercomprising: one or more dielectric layers arranged in a verticaldirection between the first curved strip and the first side region ofthe waveguide bend.
 6. The structure of claim 1 wherein the firstsection of the waveguide core is substantially straight and alignedalong a first longitudinal axis, the second section of the waveguidecore is substantially straight and aligned along a second longitudinalaxis, and the first longitudinal axis is angled relative to the secondlongitudinal axis.
 7. The structure of claim 1 wherein the secondthickness is within a range of about 0.1 to about 0.6 of the firstthickness.
 8. The structure of claim 1 wherein the first side region ofthe waveguide bend has a first width that varies as a function ofposition along the first side surface or the second side surface.
 9. Thestructure of claim 1 wherein the first side region is arranged adjacentto the central region at the first side surface.
 10. The structure ofclaim 1 wherein the first section of the waveguide core has a thirdthickness substantially equal to the first thickness of the centralregion, and the second section of the waveguide core has a fourththickness substantially equal to the first thickness of the centralregion.
 11. A method comprising: forming a waveguide core having a firstsection, a second section, and a waveguide bend that connects the firstsection with the second section, wherein the waveguide core includes afirst side surface and a second side surface, the first side surfaceextends about an inner radius of the waveguide bend and the second sidesurface extends about an outer radius of the waveguide bend, thewaveguide bend includes a central region and a first side region that isarranged adjacent to the central region at the first side surface or thesecond side surface, the central region of the waveguide bend has afirst thickness, the first side region of the waveguide bend has asecond thickness that is less than the first thickness, the first sideregion has a first end connected with the first section and a second endconnected with the second section, the first side region terminates atthe first end, and the first side region terminates at the second end;and forming a curved strip arranged over the first side region of thewaveguide bend, wherein the curved strip comprises silicon nitride, andthe waveguide bend comprises single-crystal silicon.
 12. The method ofclaim 11 wherein forming the curved strip arranged over the first sideregion of the waveguide bend comprises: forming one or more dielectriclayers arranged over the waveguide core; depositing a second layer onthe one or more dielectric layers; and patterning the second layer toform the curved strip over the first side region of the waveguide bend.13. The method of claim 11 wherein forming the waveguide core having thefirst section, the second section, and the waveguide bend that connectsthe first section with the second section comprises: patterning a firstlayer to form the waveguide core; forming an etch mask over the firstsection of the waveguide core, the second section of the waveguide core,and the central region of the waveguide bend that exposes the first sideregion of the waveguide bend adjacent to the central region of thewaveguide bend at the first side surface of the waveguide core; andetching partially through the first side region of the waveguide bendwith the etch mask over the first section of the waveguide core, thesecond section of the waveguide core, and the central region of thewaveguide bend.
 14. The method of claim 13 wherein the etch mask exposesa second side region adjacent to the central region of the waveguidebend at the second side surface of the waveguide core, and furthercomprising: etching partially through the second side region of thewaveguide bend with the etch mask over the first section of thewaveguide core, the second section of the waveguide core, and thecentral region of the waveguide bend, wherein the second side region ofthe waveguide bend has the second thickness.